1. Field of the Invention
The present invention relates to a color developer for pressure-sensitive recording paper which is excellent in color density, resistance to yellow staining, fastness to light of color images and resistance to fading in water of color images, and in addition is capable of forming color images at a markedly increased speed.
2. Prior Art
In pressure-sensitive recording paper, there is utilized in general, a combination of an electron-accepting compound (hereinafter referred to as "color developer") and an electron-donating colorless dye (hereinafter referred to as "color former"), which is dissolved in a high-boiling solvent (hereinafter referred to as "capsule oil") contained in microcapsules. When the compound and the dye are brought into contact, they undergo a reaction to form color.
Among hitherto known color developers are inorganic color developers, such as acid clay, zeolite and kaolin, and organic color developers, such as phenol compounds, novolak resins, multi-valent metal salts of aromatic carboxylic acids and multi-valent metal salts of carboxyl-modified terpene phenol resins. There has also been proposed a color developer obtainable by the co-condensation of an aromatic carboxylic acid, or an aromatic carboxylic acid-aldehyde polymer, with a xylene resin. A polyhydric metal compound can be added to this resin. Such a color developer is disclosed in Japanese Patent Publication No. 38,038/86.
Copending patent application Ser. No. 07/296,091, filed Jan. 12, 1989, now U.S. Pat. No. 4,920,186 discloses a color developer comprising a polyvalent metal-bound carboxyl-modified p-alkylphenol-mesitylene-formaldehyde co-condensate.
Copending patent application Ser. No. 07/302,242, filed Jan. 27, 1989, now U.S. Pat. No. 4,920,185 discloses a color developer comprising a polyvalent metal-bound carboxyl-modified p-substituted phenol-xylene-formaldehyde co-condensate.
Copending patent application Ser. No. 07/370,966, filed June 26, 1989, now abandoned discloses a color developer that comprises a mixture of (1) a salicylic acid-xylene-formaldehyde co-condensate resin reacted with a polyvalent metal salt and (2) a petroleum resin.
Although inorganic color developers are capable of forming color quite rapidly, they suffer from the disadvantage that their capability of forming color deteriorates during storage due to adsorption of gase and moisture from the atmosphere. Phenol compounds are inferior in their color-forming properties. Novolak-type phenol resins, in particular, p-substituted phenol-formaldehyde resins, are excellent in color-forming properties and give color images which are highly resistant to fading in water. However, novolak-type phenol resins are susceptible to yellow staining caused by light or oxidative gases (NOx, SOx, etc.) contained in the atmosphere. Multi-valent metal salts of aromatic carboxylic acids are excellent in their color forming properties and resistance to yellow staining caused by light and oxidative gases. However, color images formed in the latter system are inferior in resistance to fading in water. Multi-valent metal salts of carboxy-modified terpene-phenol resins are excellent in their color-forming properties and resistance to fading in water of color images formed. However, they suffer from the disadvantages that they are only poorly resistant to yellow staining caused by light and oxidative gases. Co-condensation products of aromatic carboxylic acids, or aromatic carboxylic acid-aldehyde polymers, with xylene resins are inferior in their color-forming properties.
As described above, color developers which have hitherto been employed suffer from various disadvantages, and hence it has been desired to improve their properties.
The purpose of this invention is to produce a color developer for pressure-sensitive recording paper which is excellent in color density, resistance to yellow staining, and fastnes to light and resistance to fading in water of color images formed, and in addition is capable of forming color images at a markedly increased speed.